Volcano Watch — Slip-sliding away—Disassembling Hawaiian volcanoes

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In 1964, irregular submarine topography north of O‘ahu and Moloka‘i was identified in newly available maps of the sea floor made by the U.S. Navy. James Moore, then Scientist-in-Charge at the USGS Hawaiian Volcano Observatory, suggested that this odd bathymetry might reflect massive landslides originating from those islands.

Moore's interpretation was disputed for more than 20 years until comprehensive mapping of the sea floor around the entire State of Hawaii was completed in the late 1980s. It turned out that Moore was right. Large—even catastrophic—submarine landslide structures litter the sea floor around the Hawaiian Islands. In fact, 17 major landslides have been identified off the shores of the main Hawaiian Islands. Fortunately, these slides are exceedingly rare—occurring, on average, only once every 350,000 years.

The largest landslides constitute significant portions of the islands from which they originated. Imagine if 10 percent of one of the islands suddenly collapsed into the ocean! Such an event would displace a huge amount of water and cause a large tsunami. Deposits of coral and sand have been found up to several hundred meters (approximately 1,000 ft) above sea level on several of the Hawaiian Islands. Catastrophic landslides are believed to have generated gigantic tsunami waves that washed ashore and left these deposits behind.

Evidence across the Hawaiian Islands suggests that landslides occur during all stages of a volcano's life. The submarine volcano Lō‘ihi—the youngest in the Hawaiian chain, located southeast of Hawai‘i Island—is characterized by a number of small landslides, even though the volcano hasn't yet breached the surface of the ocean. On the other hand, large landslides from O‘ahu and Moloka‘i clearly occurred well after the islands were established above sea level.

We also know that not all landslides in Hawai‘i are catastrophic. The south flank of Kīlauea is sliding continuously into the ocean at a rate of about 8 cm a year (3 inches a year). This motion is punctuated by large, devastating earthquakes that can cause several meters (approximately tens of feet) of seaward motion in just a few seconds (as when the magnitude-7.7 temblor struck Hawai‘i Island in 1975), as well as "slow earthquakes" (described in past Volcano Watch articles) that are associated with a few centimeters (inches) of seaward motion over the course of 1–2 days.

Will Kīlauea's south flank ever collapse suddenly? Since the shape of the south flank indicates that the slide has been active for thousands of years, there is no reason to expect that its behavior will change any time soon. Although most evidence suggests that it will continue to sag gradually, this question remains open to interpretation.

What, then, causes large landslides in Hawai‘i? Models suggest that magma pressure alone is not adequate to produce a massive landslide. One can imagine a scenario, however, in which a large eruption weakens an already unstable volcano, allowing gravity to pull the volcano apart.

Future scientific research must focus on the mechanism for giant landslides in Hawai‘i, which represent a major, if fortunately infrequent, hazard. Since other volcanic islands—such as the Canaries and the Azores—are also subject to catastrophic collapse, lessons learned from the Hawai‘i example might be fruitfully applied to mitigating hazards for the benefit of citizens elsewhere around the world.

Next week, our annual Volcano Awareness Month Volcano Watch series will conclude with an examination of questions related to volcano monitoring.

Until then, you're invited to attend a talk about Kīlauea Volcano's gas emissions and vog on January 28 in Hawai‘i Volcanoes National Park. Details are posted at http://hvo.wr.usgs.gov. You can also email askHVO@usgs.gov or call 808-967-8844 for more information.

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Volcano Activity Update

A lava lake within Halema‘uma‘u produced nighttime glow that was visible via HVO's Webcam during the past week. After nearly a month without deflation–inflation cycles (DI events) at Kīlauea's summit, a large DI event started late on January 17. In response to the deflation, the lava lake level dropped from about 45 m (148 ft) to about 70 m (230 ft) below the rim of the Overlook crater. DI inflation started on January 22 and continues as of this writing (Thursday, January 23).

On Kīlauea's East Rift Zone, the Kahauale‘a 2 flow continued to advance slowly into the forest northeast of Pu‘u ‘Ō‘ō, and satellite imagery showed the tip of the flow to have reached 7.8 km (4.8 miles) northeast of Pu‘u ‘Ō‘ō by January 22. Activity, however, waned as the DI event progressed, probably causing the most distant parts of the flow to stagnate. Webcam imagery suggests that flow activity is now waxing, with the active front slightly closer to Pu‘u ‘Ō‘ō.

There were no earthquakes reported felt on the Island of Hawai‘i in the past week.